Einstein original equations leads to expansion (or collapse) but he proposed static universe with his cosmological constant. Later on Hubble observation could be explained with expansion only, which further on proposed as accelerating expansion based on redshifts observations. This could only be spherical with visible horizon till recession velocity becomes c.

But don't let that give you the idea that there's a centre that things are moving away from. There was no big bang at a centre, from which everything then moved away. The big bang happened everywhere at once.

But don't let that give you the idea that there's a centre that things are moving away from. There was no big bang at a centre, from which everything then moved away. The big bang happened everywhere at once.

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That seems to be the case which IMO argues for initial very fast expansion from a small singularity and it does so in a wave like manner. Is there a possibility that our expansion has its own frequency and may be we are caught in one of the frequency cycles and our spacetime expansion will slow or speed up in accordance to this frequency.
There was an article on the "ringing universe" which illustrated the universe as having gone through , I believe 7 cycles of contraction and expansion during its 14.5 billion years existence. How can we tell where we are on the Pilot Wave, we may be expanding now but could there be a limit of expansion at the top of the wave and begin to decrease in the next 2 billion years ......

This is very complex. It involves dependence of electromagnetic radiation on temperature, volume and of course the constancy of entropy under adiabatic expansion. Too many assumptions.

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I had understood it was very simple: the expansion of space leads to an increase in wavelength, which is a reduction in frequency, which shifts the envelope of the black body radiation towards the red.

I had understood it was very simple: the expansion of space leads to an increase in wavelength, which is a reduction in frequency, which shifts the envelope of the black body radiation towards the red.

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So how does temperature decrease with increase in wavelength, decrease in frequency and subsequent shifting towards red ?

KittamaruAshes to ashes, dust to dust. Adieu, Sciforums.Valued Senior Member

So how does temperature decrease with increase in wavelength, decrease in frequency and subsequent shifting towards red ?

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Are you asking why the temperature decreases when there is a decrease in frequency, or are you asking the mechanism by which this occurs?

Beyond that, if the universe is, in fact, a closed system, and we know that energy can neither be created nor destroyed, but only change form, and that the universe is expanding...

Logically, it should follow, that the universe is cooling, as the energy density is decreasing (fixed value of energy against an increase value of volume, resulting in a reduction in density of energy).

Are you asking why the temperature decreases when there is a decrease in frequency, or are you asking the mechanism by which this occurs?

Beyond that, if the universe is, in fact, a closed system, and we know that energy can neither be created nor destroyed, but only change form, and that the universe is expanding...

Logically, it should follow, that the universe is cooling, as the energy density is decreasing (fixed value of energy against an increase value of volume, resulting in a reduction in density of energy).

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All fine, volume increases but how does temperature decreases in this process?

Please note that I have cautiously stayed away with a caution that it is quite complex and stated the required ingradients, but Exchemist feels that it is pretty simple as increase in wavelength etc will reduce T. How is it so simple?

PS: BTW thanks for starting to follow me. I promise you a great, objective but not necessarily pleasing to authority, learning.

I had understood it was very simple: the expansion of space leads to an increase in wavelength, which is a reduction in frequency, which shifts the envelope of the black body radiation towards the red.

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Ok the black body. But in this case who the emission source that with increasing the distance that caused the shift of frequency ?

KittamaruAshes to ashes, dust to dust. Adieu, Sciforums.Valued Senior Member

All fine, volume increases but how does temperature decreases in this process?

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If you have a set amount of energy that cannot increase, and the volume said energy is spread out across increases, the energy density will decrease. As a result, the temperature has decreased (after all, thermal emission is just a form of radiated energy).

Please note that I have cautiously stayed away with a caution that it is quite complex and stated the required ingradients, but Exchemist feels that it is pretty simple as increase in wavelength etc will reduce T. How is it so simple?

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As above - it is basic conservation of energy... unless I'm misunderstanding what it is you are questioning?

So how does temperature decrease with increase in wavelength, decrease in frequency and subsequent shifting towards red ?

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Look up black body radiation, if you do not now what that is. But in a nutshell, (a) a thermal emitter gives off radiation with a characteristic frequency distribution, and (b ) the relative amounts of the various frequencies emitted depend on the temperature of the object. This is why a heated piece of iron will, as the temperature rises be first red hot, then yellow, and then white hot.

In the case of our universe, the initial temperature of the CMBR would have been defined at the moment the universe became transparent to radiation for the first time, and it has subsequently expanded with the expansion of space.

If you have a set amount of energy that cannot increase, and the volume said energy is spread out across increases, the energy density will decrease. As a result, the temperature has decreased (after all, thermal emission is just a form of radiated energy).

Conservation of Energy at work, I believe.

As above - it is basic conservation of energy... unless I'm misunderstanding what it is you are questioning?

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Kittamaru, that's very simplistic but actually it is not. The expansion here could be isothermal or adiabatic. Say it is not isothermal, so the next step is to establish that the adiabatic expansion of the universe will decrease the temperature, to establish that for non fluid expansion of this type you have to bring in maths which will involve V and T with some power and related inversely. Can you do it for radiation? Hint : It's complex and will involve entropy too, that's why I said, it's complex.